The present invention relates to ceramic catalyst carriers and particularly to carriers for catalysts useful in the epoxidation of olefins such as for example the oxidation of ethylene to ethylene oxide, (xe2x80x9cEOxe2x80x9d). For the sake of simplicity the invention will be described in the context of this reaction but it is understood to have wider applicability.
Catalyst performance is assessed on the basis of selectivity and reactor temperature. The selectivity is the percentage of the olefin in the feed stream converted to the desired product under standard flow conditions aimed at converting a fixed percentage of the olefin in the feed stream and in the commercial production of ethylene oxide this figure is usually in the 80""s. The percentage of olefin reacted normally decreases with time and to maintain the constant level the temperature of the reaction is increased. However this adversely affects the selectivity of the conversion to the desired product. In addition, the equipment used can tolerate temperatures only up to a certain level so that it is necessary to terminate the reaction when the temperature reaches a level inappropriate for the reactor. Thus the longer the selectivity can be maintained at a high level and at an acceptably low temperature, the longer the catalyst/carrier charge can be kept in the reactor and the more product is obtained. Quite modest improvements in the maintenance of selectivity over long periods yields huge dividends in terms of process efficiency.
Epoxidation catalysts usually comprise a silver component, usually with a modifier co-deposited therewith on a ceramic carrier. It has been found that the nature of this carrier exerts a very significant influence of the performance of the catalyst carried thereon but that the reasons for that influence are not completely clear. Carriers are typically formed of a temperature resistant ceramic oxide such as alpha alumina and in general higher purity has been found to correlate with better performance. However it has been found for example that the presence of minor amounts of elemental impurities in the carrier such as alkali metals and some forms of silica can have a beneficial effect.
Intuitively it might also be considered that the higher the surface area of the carrier, the greater the area available for deposition of the catalyst and therefore the more effective the catalyst deposited thereon. This is however found not always to be the case and in modern carrier/catalyst combinations the tendency is to use a carrier with a surface area of less than 1.0 m2/g since these maintain an acceptable activity and selectivity levels while maintaining the necessary crush strength to withstand long term service in a commercial reactor without losing their physical integrity. In addition it has been found that carriers with high surface areas often have high activity but inferior selectivity.
It has now been found however that the picture with respect to carrier surface area is significantly more complicated than was at first appreciated since the nature of the porosity of the carrier has been found to play a most significant role. This discovery is the foundation for the present invention which has led to the development of a catalyst/carrier combination with excellent activity and unusually prolonged retention of a very high selectivity level at modest temperatures.
The present invention provides a carrier for an olefin epoxidation catalyst which comprises at least 95% alpha alumina with a surface area of from 1.0 to 2.6 m2/g and preferably at least 1.6 to 2.2 m2/g and a water absorption of from 35 to 55%, wherein the pores are distributed such that at least 70%, and preferably at least 80% of the pore volume is provided by pores that have pore diameters from 0.2 to 10 micrometers and provide a pore volume of at least 0.27 mL/g of the carrier. In preferred carriers according to the invention pores with diameters greater than 10 micrometers represent from 0 to 20% and preferably from 0 to 15% of the total pore volume. More preferably still pores with pore sizes less than 0.2 micrometer represent from 0 to 10% of the total pore volume. The mercury pore volume is typically up to 0.56 mL/g and more commonly from 0.35 to 0.45 mL/g.
xe2x80x9cSurface areaxe2x80x9d as the term is used herein is understood to refer to the surface area as determined by the BET (Brunauer, Emmett and Teller) method as described in Journal of the American Chemical Society 60 (1938) pp309-316. While the surface area correlates with the number and sizes of the pores and hence the pore volume, it should be noted that as a practical matter the carriers need to have a certain minimum crush strength which in turn is related to the thickness of the walls surrounding the pores. Reducing this thickness makes the walls more likely to rupture under normal loading conditions such that there is a practical limitation to the surface area of the commercially interesting carriers, at least as designed for incorporation in catalyst combinations using current technology.
The pore volume and the pore size distribution are measured by a conventional mercury intrusion device in which liquid mercury is forced into the pores of the carrier. Greater pressure is needed to force the mercury into the smaller pores and the measurement of pressure increments corresponds to volume increments in the pores penetrated and hence to the size of the pores in the incremental volume. The pore volume in the following description was determined by mercury intrusion under pressures increased by degrees to a pressure of 3.0xc3x97108 Pa using a Micromeritics Autopore 9200 model (130xc2x0 contact angle and mercury with a surface tension of 0.473 N/m).
While the pore volume of the carriers according to the invention is at least 0.27 mL/g it is preferred that pores that have pore diameters from 0.2 to 10 microns provide a pore volume between 0.30 to 0.56 mL/g to ensure that the carriers have commercially acceptable physical properties.
Water absorption is measured by measuring the weight of water that can be absorbed into the pores of the carrier as a percentage of the total weight of the carrier. As indicated above this can be in the range 35 to 55% but preferred carriers have a water absorption of 38 to 50% and most preferably from 40 to 45%.
The invention also comprises a method of making a carrier for an olefin epoxidation catalyst which comprises forming a mixture comprising:
a) from 50 to 90% by weight of a first particulate alpha alumina having an average particle size (d50) of from 10 to 90, preferably from 10 to 60, and most preferably from 20 to 40 micrometers; and
b) from 10 to 50% by weight, based on the total alpha alumina weight, of a second particulate alpha alumina having an average particle size (d50) of from 2.0 to 6.0 micrometers;
c) from 2 to 5% by weight of an alumina hydrate;
d) from 0.2 to 0.8% of an amorphous silica compound, measured as silica; and
e) from 0.05 to 0.3% of an alkali metal compound measured as the alkali metal oxide;
all percentages being based on the total alpha alumina content of the mixture, and then forming the mixture into particles and firing the particles at a temperature of from 1250 to 1470xc2x0 C. to form the carrier.
The carrier particles can be formed by any convenient conventional means such as by extrusion or molding. Where finer particles are desired these can be obtained for example by a spray drying process.
Where the particles are formed by extrusion it may be desirable to include conventional extrusion aids, optional burnout material and water. The amounts of these components to be used are to some extent interdependent and will depend on a number of factors that relate to the equipment used. However these matters are well within the general knowledge of a man skilled in the art of extruding ceramic materials.
The average particle size, referred to herein as xe2x80x9cd50xe2x80x9d, is the value as measured by a Horiba (or similar) particle size analyzer after five minutes of sonification and represents the particle diameter at which there are equal volumes of particles larger and smaller than the stated average particle size.
The method of the invention is well adapted to produce the carriers of the invention in view of the careful matching of particles sizes of the alumina components. Adjustments to the water absorption can be achieved by incorporation of conventional burnout materials which are typically finely divided organic compounds such as granulated polyolefins, particularly polyethylene and polypropylene, and walnut shell flour. However burnout material is used primarily to ensure the preservation of a porous structure during the green, (or unfired), phase in which the mixture may be shaped into particles by molding or extrusion processes. It is totally removed during the firing to produce the finished carrier. In practice the above pore size limitations mean that the carriers according to the invention do not have excessive numbers of large pores, (that is pores larger than about 10 micrometers), and have relatively few pores below 0.2 micrometer than is usually the case.
The carriers of the invention are preferably made with the inclusion of a bond material comprising silica with an alkali metal compound in sufficient amount to substantially prevent the formation of crystalline silica compounds. Typically the bond also contains a hydrated alumina component such as boehmite or gibbsite. The silica component can be a silica sol, a precipitated silica, an amorphous silica or an amorphous alkali metal silicate or aluminosilicate. The alkali metal compound can be for example a salt such as a sodium or potassium salt. A convenient bond material to be incorporated with the alumina particles used to form the carrier is a mixture of boehmite, an ammonia stabilized silica sol and a soluble sodium salt. The same effect can be achieved by incorporation of conventional ceramic bonds formulated to contain aluminosilicates and an alkali metal component. It is further found that the performance of the carrier/catalyst combination is significantly enhanced if the carrier is washed to remove soluble residues before deposition of the catalyst.